Difficulty: Introductory to Intermediate | Prerequisites: Chemistry of Life notes (bonds, water, pH). Familiarity with carbon's bonding behaviour is essential.
This is the largest content block on the exam. Every living thing is built from four classes of macromolecules: carbohydrates, lipids, proteins, and nucleic acids. Each class has its own monomers, polymers, bonding chemistry, and biological roles. You need to know not just what they are, but how they are built (dehydration synthesis), broken down (hydrolysis), and how their structure determines their function. Protein structure (four levels) and nucleic acid differences (DNA vs RNA) are especially exam-heavy.
Macromolecules are built by dehydration synthesis (removing water to link monomers) and broken down by hydrolysis (adding water to split bonds). Carbohydrates store energy (starch, glycogen) and provide structure (cellulose, chitin). Lipids include fats (energy storage), phospholipids (membranes), and steroids (signalling). Proteins have four levels of structure and lose function when denatured. DNA is double-stranded with deoxyribose and thymine; RNA is single-stranded with ribose and uracil. DNA is more stable than RNA.
Functional group
A specific grouping of atoms within a molecule that determines its chemical properties. The main ones for this course: hydroxyl (-OH, polar), carboxyl (-COOH, acidic), amino (-NH2, basic), phosphate (-PO4, acidic, involved in energy transfer), sulfhydryl (-SH, forms disulfide bridges in proteins), methyl (-CH3, nonpolar).
Dehydration synthesis (condensation reaction)
A reaction that bonds two monomers together by removing a water molecule. This is how polymers are built.
Hydrolysis
The reverse: breaking a polymer into monomers by adding a water molecule.
Monosaccharide
A simple sugar (e.g. glucose, fructose). The monomer of carbohydrates. In aqueous solution, monosaccharides form ring structures.
Polysaccharide
A polymer of many monosaccharides linked by glycosidic bonds.
Glycosidic linkage
The covalent bond between two sugar monomers. Alpha linkages (starch, glycogen) are digestible by humans. Beta linkages (cellulose) are not.
Starch
A storage polysaccharide in plants. Made of alpha-glucose. Think: potatoes, rice.
Glycogen
A storage polysaccharide in animals, stored mainly in the liver and muscles. More branched than starch.
Cellulose
A structural polysaccharide in plant cell walls. Made of beta-glucose with beta-1,4 glycosidic linkages. Humans lack the enzyme (cellulase) to digest it.
Chitin
A structural polysaccharide found in fungal cell walls and arthropod exoskeletons. Similar to cellulose but with a nitrogen-containing group.
Triglyceride (fat)
One glycerol molecule bonded to three fatty acid chains via ester linkages. Used for long-term energy storage.
Ester linkage
The covalent bond between the hydroxyl group of glycerol and the carboxyl group of a fatty acid.
Saturated fat
A fat with no double bonds in its hydrocarbon tails. Tails are straight, pack tightly, solid at room temperature. Think: butter.
Unsaturated fat
A fat with one or more double bonds, creating kinks. Does not pack tightly, liquid at room temperature. Think: olive oil.
Cis configuration
Hydrogens on the same side of the double bond. Creates a kink. This is the natural form in most biological fats.
Trans configuration
Hydrogens on opposite sides of the double bond. The chain is straighter, behaves like a saturated fat. Trans fats are produced by industrial hydrogenation and are difficult for human enzymes to break down.
Phospholipid
A lipid with a hydrophilic phosphate head and two hydrophobic fatty acid tails. The building block of cell membranes.
Steroid
A lipid built on a four-ring carbon skeleton (not fatty acid chains). Classified as a lipid because it is hydrophobic and insoluble in water. Example: cholesterol.
Amino acid
The monomer of proteins. Each has a central carbon bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen, and a variable R-group (side chain) that determines the amino acid's properties.
Peptide bond
The covalent bond between the carboxyl group of one amino acid and the amino group of another, formed by dehydration synthesis.
Primary structure
The linear sequence of amino acids in a polypeptide chain.
Secondary structure
Local folding patterns (alpha-helices, beta-pleated sheets) stabilised by hydrogen bonds between atoms of the polypeptide backbone.
Tertiary structure
The overall three-dimensional shape of a single polypeptide, determined by interactions between R-groups: hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges.
Quaternary structure
The arrangement of multiple polypeptide subunits into a functional protein. Only applies to proteins with more than one chain (e.g. haemoglobin).
Denaturation
The loss of a protein's three-dimensional shape (and therefore function) due to disruption of bonds by heat, pH change, or chemicals. The primary structure (amino acid sequence) remains intact.
Nucleotide
The monomer of nucleic acids. Composed of a five-carbon sugar (ribose or deoxyribose), a phosphate group, and a nitrogenous base.
Ribozyme
An RNA molecule that can catalyse chemical reactions. Key evidence for the RNA World hypothesis.
Functional groups are small clusters of atoms that give molecules their chemical behaviour.
Hydroxyl (-OH): polar, found in alcohols and sugars.
Carboxyl (-COOH): acidic, donates a proton to become -COO-. Found in amino acids and fatty acids.
Amino (-NH2): basic, accepts a proton to become -NH3+. Found in amino acids.
Phosphate (-PO4): highly polar, acidic, involved in energy transfer (ATP) and forms the backbone of nucleic acids.
Sulfhydryl (-SH): forms disulfide bridges (S-S bonds) in proteins, stabilising tertiary structure.
Methyl (-CH3): nonpolar. Does not participate in chemical reactions the way the others do, but affects gene expression and molecular recognition.
At physiological pH, the carboxyl group on an amino acid tends to lose a proton (becoming negatively charged) and the amino group tends to gain one (becoming positively charged), producing a zwitterion.
Monosaccharides (glucose, fructose, galactose) are simple sugars. In aqueous solution they form ring structures. They are linked into disaccharides and polysaccharides by glycosidic linkages through dehydration synthesis.
Storage polysaccharides: starch (plants, alpha-glucose) and glycogen (animals, more branched than starch).
Structural polysaccharides: cellulose (plant cell walls, beta-1,4 linkages, indigestible by humans because we lack cellulase) and chitin (fungal cell walls and arthropod exoskeletons).
Lipids are not true polymers. They are grouped together because they are hydrophobic.
Fats (triglycerides). One glycerol + three fatty acids, joined by ester linkages. Saturated fats have no double bonds and are solid at room temperature. Unsaturated fats have one or more double bonds (cis configuration causes a kink, making them liquid). Trans fats have a trans double bond that straightens the chain, making them behave like saturated fats. Human enzymes struggle to break trans fats down, which contributes to plaque buildup.
Phospholipids. Two fatty acid tails + one phosphate head. Amphipathic: the head is hydrophilic, the tails are hydrophobic. They spontaneously form bilayers in water, which is the basis of cell membranes.
Steroids. Built on a four-ring carbon skeleton (no fatty acid chains). Classified as lipids because they are hydrophobic and insoluble in water. Cholesterol is the most important steroid in this course: it is a component of animal cell membranes.
Proteins are polymers of amino acids linked by peptide bonds.
Four levels of structure:
Primary: the linear sequence of amino acids. Determined by the gene.
Secondary: local folding into alpha-helices and beta-pleated sheets, stabilised by hydrogen bonds between backbone atoms (not R-groups).
Tertiary: the overall 3D shape of a single polypeptide. Determined by R-group interactions: hydrophobic interactions, hydrogen bonds, ionic bonds, disulfide bridges.
Quaternary: the assembly of multiple polypeptide chains into one functional protein. Only relevant for multi-subunit proteins.
Denaturation is the loss of shape (and function) caused by heat, extreme pH, or chemicals. It disrupts secondary, tertiary, and quaternary structure but typically leaves primary structure (the peptide bond sequence) intact.
Exam scenario from the practice test: a single-chain protein heated slightly loses its globular shape (tertiary disrupted) but its alpha-helices remain (secondary intact). A chemical that breaks disulfide bridges and ionic bonds further disrupts tertiary structure. Quaternary does not apply because it is a single chain.
DNA and RNA are polymers of nucleotides. Each nucleotide has a five-carbon sugar, a phosphate group, and a nitrogenous base.
Two structural differences between DNA and RNA:
Sugar: DNA uses deoxyribose (missing a 2'-OH group). RNA uses ribose.
Bases: DNA uses thymine (T). RNA uses uracil (U). Both use adenine (A), guanine (G), and cytosine (C).
DNA is typically double-stranded, which provides stability through complementary base pairing and shielding of the bases. RNA is typically single-stranded, which allows it to fold into functional shapes.
DNA is more stable than RNA because (a) deoxyribose lacks the reactive 2'-OH group and (b) the double-stranded structure protects the molecule.
Ribozymes are RNA molecules that can catalyse chemical reactions. Their existence supports the RNA World hypothesis.
Students often think the methyl group (-CH3) is polar. It is nonpolar. The C-H bonds share electrons almost equally.
Students confuse which polysaccharide goes where. Cellulose is in plant cell walls, chitin is in fungal cell walls and arthropod exoskeletons. Starch is plant energy storage, glycogen is animal energy storage.
A common mistake is thinking unsaturated fats are solid. Unsaturated fats have kinks from double bonds that prevent tight packing, so they are liquid at room temperature. Saturated fats are solid.
Students mix up which level of protein structure involves R-groups. Secondary structure is stabilised by backbone hydrogen bonds. Tertiary structure is determined by R-group (side chain) interactions.
Students often think DNA contains uracil. DNA uses thymine. RNA uses uracil.
⚠️ The phosphate group question (polar, acidic, involved in ATP and nucleic acid backbone) is a favourite multiple-choice item.
⚠️ Chitin vs cellulose: both are structural, but chitin is fungi + arthropods; cellulose is plants. The exam uses this distinction.
⚠️ Cis vs trans fatty acids: know that cis creates a kink (natural), trans is straight (industrial). Trans fats are unhealthy because human enzymes cannot break them down easily.
⚠️ The protein-denaturation passage question tests whether you can identify which structural level was disrupted (tertiary, not secondary, when alpha-helices remain but the globular shape is lost).
⚠️ DNA vs RNA structural differences appear in both fill-in-the-blank (sugar, base) and short-answer format. Know why DNA is more stable (no 2'-OH, double-stranded).
⚠️ Ribozymes are tested as true/false: RNA molecules that can catalyse chemical reactions. True.
Fill in the blank: A reaction that bonds two monomers by removing a water molecule is called ______ synthesis. (Dehydration.)
Fill in the blank: The monomers of proteins are ______. (Amino acids.)
True or false: Unsaturated fats are solid at room temperature due to their straight chains. (False.)
True or false: Secondary structure in proteins is determined primarily by R-group interactions. (False. It is determined by backbone hydrogen bonds.)
Fill in the blank: The bond linking a fatty acid to glycerol is called an ______ linkage. (Ester.)
Q: Which functional group is highly polar, acidic, and involved in energy-transfer reactions like those involving ATP?
A: The phosphate group.
Q: Which structural polysaccharide is found in fungal cell walls and arthropod exoskeletons?
A: Chitin.
Q: What distinguishes a cis unsaturated fatty acid from a trans unsaturated fatty acid?
A: The orientation of hydrogen atoms around the double bond. In cis, the hydrogens are on the same side, causing a kink. In trans, they are on opposite sides, giving a straighter chain.
Q: Why is DNA more stable than RNA?
A: DNA uses deoxyribose, which lacks the reactive 2'-OH group found on ribose. DNA is also double-stranded, providing stability through base pairing and shielding of the nitrogenous bases.
Q: Describe the four levels of protein structure. In the practice exam scenario, which level was disrupted by heating and which by chemical treatment?
A: Primary (amino acid sequence), secondary (alpha-helices and beta-sheets from backbone H-bonds), tertiary (3D shape from R-group interactions), quaternary (multiple polypeptide chains). Heating disrupted tertiary structure (globular shape lost, alpha-helices intact). The chemical treatment also targeted tertiary interactions (disulfide bridges and ionic bonds). Quaternary does not apply because the protein is a single chain.
Q: What are the two components of a triglyceride and how many of each?
A: One glycerol molecule and three fatty acid chains.
Q: Why are steroids classified as lipids even though they lack fatty acid chains?
A: Because they are hydrophobic and insoluble in water, which is the defining property of lipids.
Q: Explain why trans fats are considered unhealthy.
A: Trans double bonds make the fatty acid chain straight, so it packs tightly like a saturated fat. Human enzymes did not evolve to break down this configuration efficiently, leading to accumulation and plaque buildup.
Functional groups connect directly to the chemistry of life notes (carboxyl and amino groups determine amino acid charge, which depends on pH and buffers). Protein structure connects to enzyme function: enzymes are proteins, and denaturation explains why enzymes stop working outside their optimal temperature or pH. Phospholipids and the fluid mosaic model are covered in the origin of life and cell membrane notes. Nucleic acids connect to the RNA World hypothesis and endosymbiosis (mitochondria have their own circular DNA).
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